Around one hour after midnight, put off Circle Line (8)
I believe the answer is:
diameter
'circle line' is the definition.
(I've seen this in another clue)
'around one hour after midnight put off' is the wordplay.
'around' means one lot of letters goes inside another.
'one hour after midnight' becomes 'iam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'put off' becomes 'deter' (both can mean to discourage).
'iam' inserted into 'deter' is 'DIAMETER'.
Can you help me to learn more?
(Other definitions for diameter that I've seen before include "It's equal to twice the radius" , "Line crossing centre of sphere" , "Paying back (of money)" , "Line connecting the centre of a circle with two points on the perimeter" , "A demerit for line bisecting a circle" .)